1. Field of the Invention
This present invention relates to frequency modulated transmitters and more particularly to frequency modulated stereo transmitters. 2. Description of Prior Art
Typically frequency modulated stereo transmitters have two audio stereo channels, a left and a right channel generated from a disc record or tape recording as two separate signals, one being a right signal and the other being a left signal.
Over the past several years many four-channel stereo FM transmission systems have been proposed for general broadcasting over the FM frequencies allocated by the U.S. Federal Communications Commission. The four stereo channels typically comprise a left forward channel, a right forward channel, a left back channel and a right back channel. Various four-channel FM transmission systems as well as different four-channel phonograph record systems are described in an article entitled "4-Channel FM 9 Ways to Go" by Len Feldman, in the Radio-Electronics Magazine of October 1973, starting on page 40.
As shown in FIG. 1, a block diagram of a prior art stereo FM transmission system, a two-channel stereo system, and its operation, will be described as an example. The operation of a four-channel system is analogous.
A left signal and a right signal are generated by audio equipment in a broadcast studio 10. The audio equipment will typically be a phonograph turntable playing a stereo record, a stereo tape player generating signals from stereo magnetic tape, or at least two microphones picking up music or conversation live in the studio 10. The left signal is typically inputted onto a left signal channel 12 to an audio automatic gain control (AGC) unit 14. The right signal is similarly inputted into the AGC unit 14 over a right signal channel 16. The gain controlled signals are fed into an audio limiter unit 18 where the gain controlled right signal and the gain controlled left signal are audio limited, in order to prevent overmodulation of the transmitted signal, independently of one another. The processed left and right signals are fed from the audio limiter unit 18 to a composite stereo generator 20 which generates two signals, one of which is a sum of the left and the right processed signals and the other of which is the difference of the left and right processed signals which are fed into the composite stereo generator 20 from the audio limiter 18. As is well known in the prior art, the composite stereo generator 20 typically generates the sum of the right and left signals from about 50 Hz to 15 KHz, injects a pilot carrier at 19 KHz, and generates a difference signal from about 23 KHz to 53 KHz. The output of the composite stereo generator 20 is then amplified by a transmitter unit 22 for transmission by an antenna 24.
In the prior art transmitter system, as shown in FIG. 1, the automatic gain control unit 14 limits the left and right channel signals individually. Since both the left and right signal channels must be balanced at all times in order to reproduce the input signals with fidelity, the AGC unit 14 must reduce and increase the signals passed thereby in both channels equally. Therefore, if the signal in one channel exceeds a predetermined value both signals are reduced by the same ratio which very seriously limits the modulation of the transmitter 22 and causes the well known phenomenon of stereo FM stations dropping out at a receiver before monaural FM stations having substantially the same transmitter location and transmitting power and characteristics. This dropping out phenomenon is most noticeable in mobile receivers in automobiles. Typically, prior art transmitting systems must maintain an average modulation of 70-80% in order not to be overmodulated should both the right and the left channel simultaneously generate maximum signals. Therefore, in order to foreclose the possibility of overmodulation at any time, the prior art stereo FM transmitters substantially underutilize their transmitting capacity and cover a much smaller reception area than identical monaural transmitters. This shortcoming of FM stereo transmitters has been recognized since the initiation of FM stereo broadcasting and extensive efforts have been made since then to overcome this problem.
Another problem lies in AGC units, such as 14, which typically require about fifteen frequency and amplitude filter characteristic adjustments for each channel over the audio band of frequencies which is expensive and time consuming. These filter components generate both phase distortion and lag components in the signals transmitted therethrough which impair the operation of the transmitter system by producing ringing, phase and amplitude distortion.
The audio limiter 18 also contributes to the undermodulation and amplitude distortion analogous to those contributed by the AGC unit 14. The operation of the audio limiter 18 is restricted to large amplitude and very rapidly varying signals which AGC unit 14 is not able to control. In addition, the inherent problems of balance between the left and right channel signals contribute to even more distortion since when the signal in one channel is limited, the other channel signal will almost invariably not be limited equally, thereby leading to imbalance between the channels.
In the prior art, limiting of the stereo signals is accomplished solely before a signal is generated by the composite stereo generator 20 for transmission to an antenna by the transmitter unit 22. The relationship between the sum and difference of the right and left stereo signal channels does not bear a one-to-one relationship with the individual right and left channel signals. Therefore, the gain controlled right and left signals, from the AGC unit 14 do not bear a completely predictable relationship which can be used to generate a composite stereo signal which can be adjusted in amplitude to maintain a maximally modulated transmitted signal from the antenna 24.
By limiting the left and right audio signals, the signal which modulates the transmitter unit only exercises very imprecise control of the modulation thereof which results in that the output signal from the antenna 24 varies erratically and cannot be maintained at a maximum amplitude for maximum signal coverage. In reception areas having a strong signal the automatic gain control in a typical receiver compensates for this transmitter system's shortcoming by maintaining the received volume substantially constant. However, in fringe reception areas a typical receiver is unable to control the volume which fluctuates erratically which is very annoying to a listener and makes the reception undesirable.